In the beginning, there was passion. Your feelings were almost painful. You wrote long letters and sent silly gifts and spent hours in whispered conversations on the phone. A lifetime ago. Remember?

Then came the long familiar years. You settled into a cozy, secure routine. You finished each other’s sentences; you knew the next move, the habits, the vulnerabilities, the quirks and preferences.

But what happened to the passion?

Psychotherapist Esther Perel has spent her career studying the sexual language of long-term, committed couples. She’s pondered the dynamics of the love/desire dialectic, and she’s identified the qualities that keep the sexual spark alive over the years. In a recent talk, she discussed her work with exceptional lucidity. You may intuitively know what Perel has to say, but few of us have articulated it so clearly. In any case, it’s good to be reminded—and challenged.

Desire and love are paradoxical. They’re mutually exclusive. Love, says Perel, is to have. It’s associated with security, with safety, with roots and foundations. To love is to know the beloved and to be known. But this contented intimacy isn’t a necessary component of good sex, “contrary to popular belief,” says Perel.

To desire, on the other hand, is to want. Desire craves adventure, novelty, risk. We desire mystery, the unattainable, the 50 Shades kind of guy.

\Trouble is, we want both love and desire. We want security and passion. Intimacy and mystery. Safety and risk. So how can these opposing drives coexist in a marriage? How can we settle into the mature love of a long-term relationship without losing the hungry edge of desire that brought us together in the first place? How can we achieve the ideal of a “passionate marriage,” which fans the flame of desire within the intimacy of commitment?

As she studied couples around the world, Perel asked them when they found themselves most attracted to their partner. She heard variations of the same theme:

When they reunite after an absence.

When watching the other from a distance when the partner is completely engaged in an activity. “When I look at my partner, radiant and confident, [is] probably the biggest turn-on across the board,” says Perel.

When there are no demands and no needs. “Caretaking is mightily loving,” says Perel. But, “it’s a powerful anti-aphrodisiac.”

When there is some novelty or newness. “When he’s in his tux,” said one person. Substitute cowboy boots, or a toolbelt, or motorcycle leather.

In these situations, there is a shift in perspective from the familiar to a sense of separation and distance. It’s the Proustian “voyage of discovery [that] consists, not in seeking new landscapes, but in having new eyes.”

Desire is a dialog we have with committed love. It’s a duet, a dance. The dynamic may be paradoxical, but both are necessary if a long-term relationship is to remain vital. It’s the language of poetry and mystery rather than of process and technique. Desire is more complex than bedroom gymnastics.

From her experience in studying and counseling couples, Perel has distilled several qualities that erotic couples seem to have in common. These aren’t on many “how-to” lists; they have more to do with essence than with activities. They may not be easy to incorporate because they’re not as straightforward as establishing a “date night.” But the concepts she delineates are worth some thought.

Give each other some erotic privacy. Maybe this is the space that preserves mystery. It allows the other some personal freedom to explore. It acknowledges that you aren’t joined at the hip; that there is difference and distance. “Erotic privacy may mean different things to different people,” writes Pamela Madsen, author of Shameless. “It may mean the privacy to look at pornography and not share some desires with our partners. It may mean the possibility of exploring ourselves within agreed upon boundaries without our partners.”

Foreplay isn’t optional. It isn’t a five-minute, pre-sex duty. “Foreplay pretty much starts at the end of the previous orgasm,” says Perel. These relationships cultivate a sense of erotic anticipation.

Check the “good girl” at the door. Desire is selfish. You aren’t responsible for organizing or orchestrating. “Responsibility and desire just butt heads,” says Perel.

Passion has seasons. Like the moon, it waxes and wanes. It will return, but keep on having sex in the meantime. “Willful, non-spontaneous sex,” says Madsen.